Electroluminescent displays are known as light emitting type electronic display device (ELD). An inorganic electroluminescent element (inorganic EL element) and an organic electroluminescent element (organic EL element) are cited as the constituting element of the ELD. The inorganic EL element has been used as a planar light source though high alternative voltage is required for driving such the light emitting device. The organic EL element is an element having a light emission layer placed between a cathode and an anode, in which electrons and positive holes are injected into the light emission layer and excitons are generated by recombination of them, and fluorescence or phosphorescence light is emitted on the occasion of quenching of the excitons. Such the device is noted because which can emit light by application of a voltage of several to several tens volts, and has wide viewing angle and high visibility since it is a self light emission type, and is completely solid state thin device suitable for space saving and portable appliance.
However, in an organic electroluminescence in view of the future practical application, desired has been development of an organic EL element which efficiently emits at a high luminance with a low electric consumption.
In Japanese Patent No. 3093796, a slight amount of a fluorescent substance has been doped in a stilbene derivative, distyrylarylene derivative or a tristyrylarylene derivative, to achieve improved emission luminance and a prolonged lifetime of an element. Further, there are known such as an element having an organic emission layer comprising a 8-hydroxyquinoline aluminum complex as a host compound which is doped with a slight amount of a fluorescent substance (for example, JP-A 63-264692 (hereinafter, JP-A refers to Japanese Patent Publication Open to Public Inspection No.)) and an element having an organic emission layer comprising a 8-hydroxyquinoline aluminum complex as a host compound which is doped with quinacridone type dye (for example, JP-A 3-255190).
In the case of utilizing emission from an excited singlet as described above, since a generation ratio of a singlet exciton to a triplet exciton is 1:3, that is, a generation probability of an emitting exciton species is 25% and a light taking out efficiency is approximately 20%, the limit of a quantum efficiency (next) of taking out is said to be 5%.
However, since an organic EL element which utilizes phosphorescence from an excited triplet has been reported from Princeton University (M. A. Baldo et al., Nature vol. 395, pp. 151-154 (1998)), researches on materials exhibiting phosphorescence at room temperature have come to be active.
For example, it is also disclosed in A. Baldo et al., Nature, vol. 403, No. 17, pp. 750-753 (2000), and U.S. Pat. No. 6,097,147.
Since the upper limit of internal quantum efficiency becomes 100% by utilization of an excited triplet, which is principally 4 times of the case of an excited singlet, it may be possible to achieve almost the same ability as a cooled cathode ray tube to attract attention also for an illumination application.
For example, in such as S. Lamansky et al., J. Am. Chem. Soc., vol. 123, p. 4304 (2001), many compounds mainly belonging to heavy metal complexes such as iridium complexes have been synthesized and studied.
Further, in aforesaid, A. Baldo et al., Nature, vol. 403, No. 17, pp. 750-753 (2000), utilization of tris(2-phenylpyridine)iridium as a dopant has been studied.
In addition to these, M. E. Tompson et al., at The 10th International Workshops on Inorganic and Organic Electroluminescence (EL'00, Hamamatsu), have studied to utilize L2Ir(acac) such as (ppy)2Ir(acac) as a dopant, Moon-Jae Youn. Og., Tetsuo Tsutsui et al., also at The 10th International Workshops on Inorganic and Organic Electroluminescence (EL'00, Hamamatsu), have studied utilization of such as tris(2-(p-tolyl)pyridine)iridium (Ir(ptpy)3) and tris(benzo[h]quinoline)iridium (Ir(bzq)3) (these metal complexes are generally referred to as orthometalated iridium complexes.).
Further, in also the aforesaid, S. Lamansky eat al., J. Am. Chem. Soc., vol. 123, p. 4304 (2001), studies have been carried out to prepare an element utilizing various types of iridium complexes.
Further, to obtain high emission efficiency, Ikai et al., at The 10th International Workshops on Inorganic and Organic Electroluminescence (EL'00, Hamamatsu) utilized a hole transporting compound as a host of a phosphorescent compound. Further, M. E. Tompson et al. utilized various types of electron transporting materials as a host of a phosphorescent compound doped with a new iridium complex.
An orthometalated complex provided with platinum instead of iridium as a center metal is also attracting attention. With respect to these types of complexes, many examples having a characteristic ligand are known by JP A 2002-332291, JP A 2002-332292, JP A 2002-338588, JP A 2002-226495, JP A 2002-234894, and Inorganic Chemistry, Vol. 41, No. 12, 3055-3066 (2002).
In any case, emission luminance and emission efficiency are significantly improved compared to conventional elements because the emitting light arises from phosphorescence, however, there has been a problem of a poor emission lifetime of the element compared to conventional elements. It is hard to achieve an emission of a short wavelength and an improvement of an emission lifetime of the element for a phosphorescent emission material provided with a high efficiency. At present state, it cannot be achieved a level of a practical use.
With respect to shortening of emission wavelength, heretofore, there have been known introduction of an electron attracting group such as a fluorine atom, a trifluoromethyl group, or a cyano group as a substituent group into phenylpyridine, and introduction of a ligand of such as picolinic acid or of a pyrazabole type by WO 02/15645, JP A 2003-123982, JP A 2002-11797B, JP A 2003-146996, WO 04/016711, Inorganic Chemistry, Vol. 41, No. 12, 3055-3066 (2002), Applied Physics Letters Vol. 79, 2082 (2001), Applied Physics Letters Vol. 83, 3818 (2003), and New Journal of Chemistry, vol. 26, 1171, (2002). However, when an emission wavelength is shortened to achieve blue color by utilizing these substitution effects, a high efficiency may be achieved while emission lifetime will be greatly deteriorated, which requires further improvement to overcome the trade-off relationship.
Light emitting material having high phosphorescence efficiency is difficult to make the emission light wavelength shorter and improve emission lifetime, and therefore, it does not display performance sufficient for practical use.
A metal complex having phenyl pyrazole as a ligand, for example, patent documents 1 and 2. While the substitution manner of phenyl pyrazole with phenyl group is improved in emission lifetime, it is not sufficient and there is a room to improve emission efficiency.
On the other hand, the vapor deposition process which is usually applied in the production of the organic EL device using a low molecular weight compound causes problems of production equipment and energy efficiency when the organic EL element having enlarged area is manufactured, and it is thought that a printing method including a ink-jet printing method and a screen printing method or a coating method such as a spin coating method and a cast coating method are desirable. Though an organic metal complex having dendrimer portion (see, Patent Document 3), and an organic metal complex fixed in polymer chain (see, Patent Document 4) are known as a phosphorescence material suitable for printing method or coating method, they are insufficient and improvement is desired in view of emission efficiency or life time.    Patent Document 1: WO 04/085450    Patent Document 2: JP A 2005-053912    Patent Document 3: WO 02/066552    Patent Document 4: JP A 2003-342235